Volcano eruptions are both complex and exciting as they display a variety of eruption style depending on the form and the shape related to each type of volcano. Each characteristic has its own effect on the environment. Also, the volcanic activity level is determined by the volcano eruptions.

Thursday, May 19, 2005

Mount St.Helen's history

Mount St Helens is an active stratovolcano in Skamania County, Washington, in the Pacific Northwest region of the United States. It is located 96 miles (154 km) south of Seattle and 53 miles (85 km) northeast of Portland, Oregon. The mountain is part of the Cascade Range and was initially known as Louwala-Clough which means "smoking or fire mountain" in the language of the Klickitats. It was named for British diplomat 1st Baron St Helens as a result of a late 18th century British survey of the area. It is most famous for a catastrophic eruption on May 18, 1980. That eruption was the most deadly and economically destructive volcanic eruption in the history of the United States. Fifty seven people were killed and 200 homes, 47 bridges, 15 miles (24 km) of railways and 185 miles (300 km) of highway were destroyed. The eruption blew off the top of the mountain, reducing its summit from 9,677 feet (2,950 m) to 8,364 feet (2,550 m) in elevation and replacing it with a mile-wide (1.5 km-wide) horseshoe-shaped crater (see geology section or 1980 eruption of Mount St. Helens for more detail). Like most other volcanoes in the Cascade Range, St. Helens is a great cone of rubble consisting of lava rock interlayered with ash, pumice and other deposits. Volcanic cones of this internal structure are called composite cones or stratovolcanoes. Mount St. Helens includes layers of basalt and andesite through which several domes of dacite lava have erupted. The largest of the dacite domes formed the previous summit; another formed Goat Rocks dome on the northern flank. These were destroyed in St. Helens' 1980 eruption. Mount St. Helens is 34 miles (55 km) almost due west of Mount Adams, which is in the eastern part of the Cascade Range. These "sister and brother" volcanic mountains are each about 50 miles (80 km) from Mount Rainier, the giant of Cascade volcanoes. Mount Hood, the nearest major volcanic peak in Oregon, is about 60 miles (95 km) southeast of Mount St. Helens. Even before its loss of height, Mount St. Helens was not one of the highest peaks in the Cascade Range. Its summit altitude made it only the fifth highest peak in Washington. It stood out prominently, however, from surrounding hills because it rose thousands of feet above them and had a perennial cover of ice and snow. The peak rose more than 5,000 feet (1500 m) above its base, where the lower flanks merge with adjacent ridges. The mountain is about 6 miles (9.5 km) across at its base which is at an altitude of about 4,400 feet (1340 m) on the northeastern side and about 4,000 feet (1220 m) elsewhere. At the pre-eruption timberline (upper limit of trees) the width of the cone was about 4 miles (6.4 km). Streams that head on the volcano enter three main river systems — the Toutle River on the north and north-west, the Kalama River on the west, and the Lewis River on the south and east. The streams are fed by abundant rain and snow that dump an average of about 140 inches (3.6 m) of water on Mount St. Helens a year, according to National Weather Service data. The Lewis River is impounded by three dams for hydroelectric power generation. The southern and eastern sides of the volcano drain into an upstream impoundment, the Swift Reservoir, which is directly south of the volcano. Although Mount St. Helens is in Skamania County, Washington the best access routes to the mountain run through Cowlitz County, Washington on the west. Washington State Route 504, locally known as the Spirit Lake Memorial Highway, connects with the heavily traveled Interstate 5 at Exit 49, about 34 miles (55 km) to the west of the mountain. That major north-south highway skirts the low-lying cities of Castle Rock, Longview and Kelso along the Cowlitz River and passes through Vancouver, Washington-Portland, Oregon, metropolitan area less than 50 miles (80 km) to the southwest. The community nearest the volcano is Cougar, Washington which is in the Lewis River valley about 11 miles (18 km) south-southwest of the peak. Gifford Pinchot National Forest surrounds Mount St. Helens, but some land owned by Washington is in private hands. Native Americans in the area developed various myths and legends surrounding St. Helens. Perhaps the most famous of these is the Bridge of the Gods legend told by the Klickitats. In their tale, the beautiful maiden Loowit and the two sons of the Great Spirit Sahale, Pahto (also called Klickitat) and Wyest, were the three guardians of the Columbia River. Pahto ruled north of the Bridge of the Gods and Wyest ruled south of it. Both chiefs loved the undecided Tah-one-lat-clah and waged war over her by throwing fire and stones across the river. The area was devastated and the earth shook so violently that the huge bridge fell into the river, creating the cascades of the Columbia River Gorge. For punishment, Sahale struck down each of the lovers and transformed them into great mountains where they fell. Wyeast, with his head lifted in pride, became the volcano known today as Mount Hood and Pahto, with his head bent toward his fallen love, was turned into Mount Adams. The fair Loowit became Mount St. Helens, known to the Klickitats as Louwala-Clough which means "smoking or fire mountain" in their language. The first recorded sighting of Mount St. Helens by Europeans was by Royal Navy Commander George Vancouver and the officers of HMS Discovery on May 19, 1792, while they were surveying the northern Pacific Ocean coast from 1792 to 1794. Vancouver named the mountain for British diplomat Alleyne Fitzherbert, Baron St. Helens on October 20, 1792, as it came into view when the Discovery passed into the mouth of the Columbia River. Strictly, the name of the volcano should not be written as "Saint Helens", since that is not the name of the Baron. Years later, the mountain was visited by its first major eruption after explorers, traders, and missionaries heard reports of an erupting volcano in the area. Much later geologists and historians determined that the eruption took place in 1800 and was the start of the 57 year long Goat Rocks Eruptive Period (see geology section below). Alarmed by the "dry snow", the Nespelim of northeastern Washington spent a great deal of time in prayer and dance instead of collecting food for winter. They therefore had a hard winter. In 1805 and 1806 members of the Lewis and Clark Expedition spotted St. Helens from the Columbia River but did not report any eruption in progress or recent evidence of one. They did report the presence of quicksand and clogged channel conditions at the mouth of the Sandy River near Portland, suggesting an eruption by Mount Hood sometime in the previous decades. The first authenticated eyewitness report of a St. Helens eruption was made in March 1835 by Dr. Meredith Gairdner, then working for the Hudson's Bay Company stationed at Fort Vancouver (the first geologist apparently viewed the volcano 6 years later). He sent an account to the Edinburgh New Philosophical Journal, which published his letter in January 1836. James Dwight Dana of Yale University, while sailing with the Charles Wilkes U.S. Exploring Expedition, saw the peak (then quiescent) from off the mouth of the Columbia River in 1841. Another member of the expedition later described "cellular basaltic lavas" at the mountain's base. In either late fall or early winter 1842 the so-called "Great Eruption" was seen by settlers and missionaries in the area. Large ash clouds were reported for this small volume outburst and mild explosions followed for 15 years. All these eruptions were likely phreatic (steam explosions). The Reverend Josiah Parrish in Champoeg, Oregon witnessed Mount St. Helens in eruption on November 22, 1842. Ash from this eruption may have reached The Dalles, Oregon 48 miles (80 km) southeast of the volcano. Future California governor Peter H. Burnett in October 1843 recounted a story of a Native American man who badly burned his foot and leg in either lava or hot ash while hunting for deer. The story went that the injured man sought treatment at Fort Vancouver but the contemporary fort commissary steward, Napolean McGilvery, disclaimed knowledge of the incident. British lieutenant Henry J. Warre sketched the eruption in 1845 and the same year Canadian painter Paul Kane created watercolors of some of the outbursts. Both men's work showed erupting material from a vent about a third of the way down from the summit on the mountain's west or northwest side (possibly at Goat Rocks). On April 17, 1857 the Republican, a Steilacoom, Washington newspaper, reported that "Mount St. Helens, or some other mount to the southward, is seen . . . to be in a state of eruption". The lack of a significant ash layer associated with this event indicates that it was a small eruption, which may have been nothing more than billowing clouds of steam and dust. This was the first reported activity from the volcano since 1854 and the last until 1980. During the lead-up to the 1980 Mount St. Helens eruption, 84 year old innkeeper Harry Truman, who had lived near the mountain for over 50 years, became nationally famous when he decided not to evacuate before the impending eruption, despite repeated pleas by local authorities. His body was never found after the May 18, 1980 eruption, which left a huge crater open to the north (see geology section below). In total, 57 people were killed or never found. Had the eruption occurred one day later, when loggers would have been at work, rather than on a Sunday, the death toll would almost certainly have been much higher. U.S. President Jimmy Carter surveyed the damage and stated it looked more desolate than a moonscape. A film crew was dropped by helicopter on St. Helens on May 23 to document the destruction. Their compasses, however, spun in circles and they quickly became lost. A second eruption occurred the next day but the crew survived and were rescued two days after that. In 1982, President Ronald Reagan and the U.S. Congress designated a 110,000 acre (445 km2) area around the mountain the Mount St. Helens National Volcanic Monument. The Monument resides inside Gifford Pinchot National Forest. The area is being left to gradually return to its natural state following the devastation. According to geological evidence, St. Helens started growth in the Pleistocene 37,600 years ago with dacite and andesite eruptions of pumice and ash. 36,000 years ago a large mudflow cascaded down the volcano (mudflows were very significant forces in all of St. Helens' eruptive cycles). Parts of this ancestral cone were fragmented and transported by glaciers 14,000 to 18,000 years ago during the last ice age. Repeated eruptions of pyroclastic flows, pumice, and ash followed until about 6500 BC when the volcano went dormant for 4000 years. Starting around 2500 BC eruptions of large amounts of ash and yellowish-brown pumice covered thousands of square miles. This eruptive cycle lasted until about 1600 BC and left 18 inch (46 cm) deep deposits of material 50 miles (80 km) distant in what is now Mt. Rainier National Park and trace amounts have been found as far northwest as Banff National Park in Alberta and as far southeast as eastern Oregon. All told there may have been up to 2.5 cubic miles (10 km³) of material ejected in this cycle. After 400 years of inactivity, St. Helens came alive again around 1200 BC. This cycle, which lasted until about 800 BC, is characterized by smaller volume eruptions. Numerous dense nearly red hot pyroclastic flows sped down St. Helens' flanks and came to rest in nearby valleys. A large mudflow partly filled 40 miles (65 km) of the Lewis River valley sometime between 1000 BC to 500 BC. The next eruptive cycle began roughly around 400 BC and is characterized by a change in composition of St. Helens' lava, which diversified by adding olivine and basalt to the mix. Also different was the presence of significant lava flows in addition to the previously much more common fragmented and pulverized lavas and rocks (tephra). Large lava flows of andesite and basalt covered parts of the mountain, including one around the year 100 that traveled all the way into the Lewis and Kalama river valleys. Others, such as Cave Basalt (known for its system of lava tubes), flowed up to 8 to 9 miles (13 to 15 km) from their vents. Also around the 1st century, mudflows moved 30 miles (50 km) down Toutle and Kalama river valleys and may have reached the Columbia River. Another 400 or so years of dormancy ensued. Sometime around the year 500 small quantities of ash and lava erupted from St. Helens' north flank. This period ended with the emplacement of dacite domes, including Sugar Bowl around the year 800. Roughly 700 years of dormancy was broken about the year 1500 when large amounts of pale gray dacite pumice and ash started to erupt in the Kalama eruptive cycle. At least seven different beds were laid down in the most voluminous eruptive cycle for 3000 years. Ash and pumice piled to a thickness of three feet (1 m) six miles (9.5 km) northeast from the volcano and two inches (5 cm) deep 50 miles (80 km) away in the same direction. Large pyroclastic flows and mudflows subsequently rushed down St. Helens' west flanks and into the Kalama River drainage system. The source for at least some of these debris flows may have come from the destruction of a dacite dome close to or at the summit. The next phase of this 150 year long cycle saw the eruption of less silica-rich lava in the form of andesitic ash that formed at least eight alternating light and dark-colored layers of ash. After that, blocky andesite lava flowed from St. Helens' summit crater down the volcano's southeast flank. Later, pyroclastic flows raced down over the andesite lava and into the Kalama River valley. This cycle ended with the emplacement of a large dacite dome at the volcano's summit. The several hundred foot high dome filled and overtopped an explosion crater already at the summit. Large parts of the dome's sides broke away and mantled parts of the volcano's cone with talus. Lateral explosions excavated a notch in southeast crater wall. St. Helens reached its greatest height and highly symmetrical form by the time the Kalama eruptive cycle ended on or around 1647. 150 years of quiet returned to the volcano. The 57 year long Goat Rocks Eruptive Period started in 1800 and is the first cycle for which oral and written records exist. As with the Kalama cycle, the sequence of events started with an explosion of dacite tephra followed by an andesite lava flow and then culminated with the emplacement of a dacite dome. The ash drifted northeast over central and eastern Washington, northern Idaho and western Montana. There were at least a dozen small eruptions between 1831 to 1857 of ash reported as well. The vent apparently was at or near Goat Rocks.